Barack Obama heralded a "bold new vision" for human spaceflight tonight, using the symbolic backdrop of Cape Canaveral, America's gateway to the stars, to try to rescue his strategy for the next generation of exploration and discovery.

In recent days some heavyweight critics, including Neil Armstrong, the first man on the moon, and several fellow Apollo-era astronauts, have criticised the US president for abandoning the $108bn (£70bn) Constellation programme of rockets and spacecraft designed to return man to the lunar surface by 2020.

But in an address at Florida's Kennedy Space Centre, the launch site of every manned US mission since 1961, and accompanied by Armstrong's Apollo 11 crewmate Buzz Aldrin, a key supporter, Obama attempted to quell the growing dissent by unveiling an "ambitious" new direction for his administration's space policy.

The plan includes an extra $6bn for Nasa over five years; $3.1bn for "vigorous new technology development" that could result in a pioneering heavy-lift rocket to take astronauts to Mars, with future "stepping stone" missions into deep space to help get them there; support for those losing jobs when the 30-year-old space shuttle programme ends this year and the salvaging of Constellation's axed crew capsule for use at the international space station.

He said he expected the first crewed missions beyond the moon by about 2025, and to orbit Mars by the middle of the following decade. "A landing on Mars will follow. I expect to be around to see it," he said.

"What we're looking for is not to continue on the same path, but to leap into the future. Pursuing this new strategy will require that we revise the old strategy, in part because it was not fulfilling its promise in many ways. We can't keep doing the same old things we've been doing and think it'll get us where we need to go. We will reach space faster and more often under this new plan."

It remains to be seen if Obama's criticsin the space community, and on the US Senate floor, will be appeased. The financial content of the speech, such as a proposal to expand Nasa's funding to $100bn over the next five years, and money for commercial enterprises to develop private space "taxis" for cargo and crew, was announced in the president's budget proposal to Congress in February.

There will be doubt over the real value to Nasa of some new elements, specifically the cherry-picking from Constellation of the Orion capsule to act as an "escape boat" for crew aboard the space station in an emergency.

In the development of Constellation, which cost $9.1bn in five years before it was axed on cost grounds, Orion was designed as the flagship spacecraft that would carry astronauts to the space station and eventually the moon. Its reprieve, Obama said, would "establish a technological foundation for future exploration spacecraft and preserve hi-tech contractor jobs in Colorado, Texas and Florida."

Yet Nasa will still have to rely on Russian Soyuz craft to take American astronauts to the space station until commercial spaceships are ready, a main criticism of Obama's opponents.

Obama believes that investment in the commercial sector will provide more than 10,000 new jobs nationally, but detractors point out that Constellation was expected to provide 25,000 jobs in its lifetime.

Charles Bolden, the former shuttle astronaut appointed by Obama to lead Nasa this year, disagrees with the critics, among them four of the 12 men to have walked on the moon.

Apollo legends Armstrong, Charles Duke, Harrison Schmitt and Eugene Cernan signed open letters this week condemning Obama for risking ceding US superiority in space and leading its space programme on "a long, downhill slide to mediocrity".

Bolden said: "The fundamental goal has not changed – to boldly advance human presence beyond the cradle of Earth. President Barack Obama is strongly committed to our future in space."

• This article was amended on 16 April 2010. In the original Neil Armstrong and Buzz Aldrin were described as crewmates on Apollo 13. They were crewmates on Apollo 11. This has been corrected.